Aqueous resin compositions are supplied in the form of emulsion composition, for instance, and by constituting water-based coatings, find application in a variety of uses; however, when such compositions are to be used in water-based clear coating compositions for automotive use, for instance, they are required to provide for excellent appearances, favorable coating film properties and availability at low cost. It should also be understood that such aqueous resin compositions are advantageous from the standpoint of cutting on volatile organic compounds (VOC).
The prior art includes the following aqueous resin compositions.
Japanese Kokai Publication Sho-50-158682 (pages 455 and 457) discloses a process for producing a vinyl polymer, which comprises suspension-polymerizing a vinyl monomer in the presence of α-methylstyrene and/or its dimer. However, precipitation of particles occurs in a water-based coating comprising the suspension polymer composition with lapse of time and some further technical sophistication is required to improve the storage stability.
Japanese Kokai Publication Hei-3-109451 (page 421) discloses a process for producing a copolymer latex which comprises emulsion-polymerizing a monomeric material comprising a conjugated diene monomer, an ethylenically unsaturated monomer and an ethylenically unsaturated carboxylic acid monomer in the presence of α-methylstyrene dimer. However, since the copolymer latex prepared by this process cannot form a coating film through crosslinking, some further technical sophistication is required to insure expression of satisfactory coating film properties such as those required of water-based clear coating compositions for automotive use.
Japanese Kokai Publication Hei-7-70431 (pages 1 to 2) discloses a synthetic resin aqueous dispersion containing (A) an ionic resin, (B) a blocked polyisocyanate and (C) a polymer formed from, by free radial polymerization, at least one kind of ethylenically unsaturated monomer. Referring to this synthetic resin aqueous dispersion, an amino-epoxy resin is mentioned as an example of the ionic resin and the dispersion is used in electrocoating materials but since the electrocoating material is an undercoating for the automotive coating, some further technological sophistication is needed for expression of the excellent appearance and good coating film properties such as those required of a water-based coating composition for use as a top coating.
Japanese Kokai Publication Hei-8-59932 (pages 1 to 2) discloses an aqueous dispersion composition which comprises an acrylic copolymer emulsion containing an acrylic copolymer, a water-soluble high polymer, a polyhydric alcohol, and a polyvalent metal compound in a defined amount. Moreover, Japanese Kokai Publication Hei-8-259613 (pages 1 to 3) discloses an anionic polymer blend having a reduced potential of bubble formation in water which comprises an acidic group-containing polymer A and a water-insoluble polymer B in the form of latex particles and being able to be produced by the emulsion polymerization of an ethylenically unsaturated compound in the presence of the polymer A. However, these compositions are intended to provide binders for water-based printing inks or an acidic group-containing polymer blend to be used as a binder for water-based print-coating and neither of them is capable of forming a coating film through crosslinking, with the result that a further technological sophistication is needed in order that they may contribute to the expression of excellent appearances and coating film properties when used in water-based coatings and the like.
Japanese Kokai Publication Hei-11-131017 (pages 2 and 12) discloses an aqueous cladding composition which comprises (i) a polyol component comprising one member selected from the group consisting of those polyesters, polyacrylates, and polyester polyacrylates which contain a hydroxyl group and a carboxyl group and have weight average molecular weights within the range of 2000 to 50000 and glass transition temperatures not below 30° C. and (ii) a polyisocyanate component comprising one member selected from the group consisting of those aliphatic isocyanates and alicyclic isocyanates which have blocked isocyanate groups. However, since this aqueous cladding composition is prepared by polymerization in a solvent, phase inversion, and desolvation in that sequence, the production process involves many steps and, hence, has room for further technological sophistication for cost reduction.
Japanese Kokai Publication Hei-7-102206 (pages 2 and 6 to 8) is directed to a water-based coating composition predominantly composed of a thermosetting resin composition comprising a crosslinking agent and an aqueous dispersion of the acrylic resin obtainable by, in an organic solvent, solution-polymerizing an acidic group-containing polymerizable unsaturated monomer, a hydroxyl group-containing polymerizable unsaturated monomer, a styrenic monomer, and one or more other polymerizable unsaturated monomers, an aqueous acrylic resin solution obtainable by solution-polymerizing the corresponding monomer in an organic solvent and converting the resulting polymer solution to an aqueous solution, in which water-based coating composition, melamine and blocked isocyanates are mentioned as species of the crosslinking agent. However, when the acrylic resin is to be provided in the form of an aqueous dispersion, a further technological sophistication is required to improve coating film properties in order that the resulting water-based coating composition may be suitable for clear coating compositions for automotive use. On the other hand, when the acrylic resin is to be provided in the form of an aqueous solution, the process inevitably involves a step for conversion to an aqueous solution so that further ingenuity is required for reducing the production cost through curtailment of the process flow or reducing the VOC level in the production stage.
Japanese Kokai Publication Hei-11-131017 referred to above (page 2) discloses a aqueous cladding composition which comprises a polyol component comprising one member selected from the group consisting of polyesters, polyacrylates, and polyester polyacrylate polyols and a polyisocyanate component comprising one member selected from the group consisting of aliphatic isocyanates and alicyclic isocyanates and a process for producing said composition. Furthermore, Japanese Kokai Publication 2002-105396 (pages 2 and 5 to 6) is directed to a water-based coating composition characterized by its containing an isocyanate compound having at least two blocked isocyanate groups in the molecular and an aminoplast resin series crosslinking agent having a methylol group or a methylol ether group in the film-forming component, said film-forming component being dispersed in water with the aid of a suspension stabilizer, in which it is disclosed that the aminoplast resin includes an N-butoxymethylacrylamide-containing resin and that the suspension stabilizer includes a hydroxyl group-containing, high-acid-value resin. However, these processes invariably involve a step for conversion to an aqueous solution or a step for removing the solvent after formation of the water-based coating composition and, therefore, some technological innovation is needed for curtailing the process flow to thereby reduce the production cost and reducing the VOC level in the production stage.